This invention relates, in general, to GaAs heterostructure devices, and more particularly, to pseudomorphic GaAs MODFETs having a stress-compensating layer.
Pseudomorphic AlGaAs/In.sub.x Ga.sub.1-x As/GaAs MODFETs provide better high-frequency performance in Comparison to conventional AlGaAs/GaAs MODFETs. Pseudomorphic growth, or coherently strained growth of epitaxial layers on the GaAs substrate, introduces little or no misfit dislocations that degrade device performance. The high frequency and high speed performance of MODFETs improves as x increases in the In.sub.x Ga.sub.1-x As channel layer. This is due to the fact that the electron transport properties of In.sub.x Ga.sub.1-x As improve with increasing x. However, the lattice mismatch between In.sub.x Ga.sub.1-x As and GaAs increases with x. The stress and formation of misfit dislocations due to the large lattice mismatch degrades the material quality of the In.sub.x Ga.sub.1-x As channel layer, which results in a degradation of device performance. For the heterostructure to be pseudomorphic and dislocation-free, the thickness of the InxGal-xAs channel layer has to stay below a critical value for a given x value.
A way of increasing the value of x or reducing the stress is to reduce the thickness of the channel layer, however, the channel layer can only be reduced to a certain thickness before electrical characteristics will be adversely affected. The practical channel thickness requirement sets an upper limit of x approximately equal to 0.25 for high quality devices. It would be desirable to be able to increase x without degrading electrical characteristics.
A process that would reduce the lattice mismatch strain would allow the use of higher x values. One such process entails growing a graded In.sub.x Ga.sub.1-x As layer that is thicker than the critical thickness to purposely form dislocations far enough away from the active channel region. In that case, dislocations and related defects do not degrade the properties of the active region of the channel layer. The disadvantage of this process is that a very thick In.sub.x Ga.sub.1-x As layer must be grown to keep defects away from the active channel region. Even so, there is no guarantee that the defects will not propagate to the channel. In addition, from a production point of view, the growth of a graded layer is disadvantageous, because it is less cost-effective.
An InP MODFET has been proposed by Albert Chin and T. Y. Chang in an article entitled, "Achievement of Exceptionally High Mobilities in Modulation-doped Ga.sub.1-x In.sub.x As on InP Using a Stress Compensated Structure," published in the Journal of Vacuum Science Technology, Vol. B 8(2), pp. 364-366 (1990). For the growth of In.sub.x Ga.sub.1-x As on InP substrates, the value of x that gives lattice matching is 0.53. Again, it is desirable to increase x for the enhancement of device performance, but limitations similar to those in the case of GaAs also apply. In the InP structure of Chin and Chang, an In.sub.0.25 Ga.sub.0.75 As layer is formed adjacent to the In.sub.x Ga.sub.1-x As channel layer, with x greater than 0.53 in order to compensate for stress. The lattice constant for ln0.25Ga0.75As is less than the lattice constant for InP. Thus, the strain is opposite in sign to that in a layer of In.sub.x Ga.sub.1-x As with x greater than 0.53. Unfortunately, this approach of varying x in adjacent layers is not applicable for growth on GaAs substrates, because the lattice constant of In.sub.x Ga.sub.1-x As is greater than the lattice constant of GaAs for all values Of x greater than 0. InP substrates are fragile and expensive in comparison to GaAs substrates, thus, it would be desirable to improve the materials quality of pseudomorphic heterostructures on GaAs substrates to give higher device performance and reliability.
Accordingly, it is an object of the present invention is to provide an improved GaAs MODFET.
Another object of the present invention is to provide a GaAs MODFET having high frequency and high speed performance.
A further object of the present invention is to provide a GaAs MODFET having an increased mole fraction of InAs in the channel layer.
Yet another object of the present invention is to provide a pseudomorphic AlGaAs/InGaAs/GaAs MODFET having a channel layer having an optimized thickness and mole fraction of InAs and little or no misfit dislocations.